1. Field of the Invention
The present invention relates to a rotary actuator for rotating a driven member, particularly suited for high load applications.
2. Description of the Related Art
Known techniques relating to rotary actuators include the following:
1) Driving a driven member directly with the output shaft of a motor as shown, for example, in Japanese Patent Laid-Open Publication No. 2001-271925;
2) Increasing the output torque of a motor with a worm and wheel set and driving a driven member with the output shaft of the worm reduction gear as shown, for example, in Japanese Patent Laid-Open Publication No. Hei 5-60217; and
3) Increasing the output torque of a motor with a spur gear set and driving a driven member with the output shaft of the spur reduction gear.
With the prior art 1 device above, the motor size increases as the load increases, because of the structure that drives the driven member directly with the output shaft of the motor. For example, in high torque applications such as a servo mechanism for the switching operation of a parking lock and shift range selector of a vehicle, the motor according to the prior art 1 will have to be substantially large. However, the spacing between the automatic transmission and vehicle body is usually as small as 40 to 50 mm for more interior space, and a bulky motor in such an application is problematic. That is, the vehicle body design may have to be changed so as to accommodate the large motor, resulting in smaller interior space.
The prior art 2 device above requires a damper or the like to prevent a lock between the worm and wheel in the worm reduction gear. The worm and wheel engage each other in a small number of points in this reduction gear, and in order to accommodate higher loads, the system needs to be larger. If this system is to be used for the aforementioned servo mechanism in a vehicle which experiences high loads, it will present the same problems as noted above because of its size, i.e., it will hardly be mountable in the small space between the automatic transmission and vehicle body, and the vehicle body design may have to be changed, resulting in less interior space.
The prior art 3 device above also needs to be larger to accommodate higher loads for the same reason as noted above, because the plurality of spur gears engage each other in a small number of points. If this system is to be used for the aforementioned servo mechanism in a vehicle which experiences high loads, it will present the same problems as noted above because of its size, i.e., it will be difficult to mount in the small space between the automatic transmission and vehicle body, and the vehicle body design may have to be changed, resulting in a smaller interior space.